1. Field of the Invention
This invention relates to metal grinding machines and more particularly to a grinding machine for automatically or manually removing a surface layer of material from elongated metal workpieces in preparation for a subsequent operation.
2. Description of the Prior Art
Semi-finished, elongated workpieces such as steel slabs or billets are invariably coated with a fairly thin layer of oxides or other impurities which may extend into the billet a considerable distance, and defects consisting usually of longitudinal cracks at localized points on the surface of the billets. These impurities must be removed before the billets are rolled into finished products since the impurities and defects would otherwise appear in the finished product. Cracks particularly must be removed as subsequent operations invariably enlarge them. Billet grinders utilizing a reciprocating car for moving the billet longitudinally beneath a rotating grinding wheel or for moving the grinding wheel longitudinally above the billet have long been used to perform these functions. The relatively thin layer is removed by a "skinning" procedure in which the billet reciprocates beneath the grinding wheel with the grinding wheel moving transversely after each reciprocation or grinding pass until the entire surface of the billet has been covered. Relatively deep impurities and defects are then visually apparent, and they are removed by a "spotting" procedure in which the grinding wheel is held in contact with the localized area until all of the impurities have been removed.
Various techniques have been devised to automate the skinning procedure by automatically reciprocating the billet beneath the grinding wheel and moving the grinding wheel transversely an incremental distance each grinding pass until the entire surface has been covered. The basic problem with these systems has been their inability to remove a constant depth of material at a rapid rate particularly from non-straight workpiece surfaces thus either severely limiting the speed at which workpieces are conditioned or removing an excess quantity of metal from workpieces. These problems are principally due to excessive wheel vibration caused by wear resulting from exposure of the sliding ways to an abrasive environment and the use of control systems having a relatively slow response time which are thus incapable of responding to irregular workpiece surfaces at a sufficient rate.
One very sophisticated, microprocessor based grinding system is disclosed in U.S. patent application Ser. No. 748,293, now U.S. Pat. No. 4,100,700 issued July 18, 1979. Basically, this system computes the power required to produce a predetermined depth of cut of a predetermined width at a given car velocity. The calculated power is then compared with the actual rotational velocity of the grinding wheel to derive a torque command which is compared to the actual motor torque to produce a control signal for raising and lowering the grinding wheel from the workpiece.
Although grinding systems have been used which attempt to maintain the grinding pressure substantially constant, they have not proved satisfactory in actual use. These prior art systems generally utilize fairly light grinding heads which tend to vibrate excessively with detrimental effects upon wheel wear and life. Use of massive grinding heads has not been possible because conventional closed loop control techniques for controlling the grinding force are unable to operate with massive heads without excessive phase shifts which may cause the system to become unstable under certain conditions.